This invention relates to an inspecting apparatus for use in inspecting an object to detect a defect on a surface of the object. It is to be noted throughout the instant specification that description will be mainly made about a photomask blank, although this invention is applicable to a photomask or the like.
A photomask blank comprises a transparent substrate and a shading film deposited over one entire surface of the substrate. Such a photomask blank plays a very important role in fabricating a semiconductor integrated circuit device by the use of a photolithography technique. This means that defects in the photomask blank bring about serious influences on a yield of the semiconductor integrated circuit device. Accordingly, such defects on the photomask blank must be inspected and reduced to a minimum.
It is mentioned here that such defects can be classified into pinhole defects and particle defects. The pinhole defects result from undesired pinholes formed on the shading film of the photomask blank. The particle defects result from particles, such as dust and the like, undesirably adhered to the shading film of the photomask blank. The pinhole defects are more serious than the particle defects in the photomask blank because the pinhole defects directly give rise to deterioration of the yield of the semiconductor integrated circuit device. Therefore, the pinhole defects should be accurately inspected in comparison with the particle defects.
In order to inspect or detect such pinhole defects and particle defects, an inspecting apparatus has been sold by Hitachi Denshi Engineering Co., Ltd. In this inspecting apparatus, a photomask blank is scanned by a light beam of a particular wavelength to detect a reflected light beam and a transmitted light beam which are reflected on and transmitted through the photomask blank. The reflected light beam is scattered by the particle defects and results in a variation of intensity. Therefore, it is possible to detect the particle defects by monitoring the variation of intensity in the reflected light beam. On the other hand, the pinhole defects can be detected by monitoring the transmitted light beam.
According to the inventor's experimental studies, it has been found that an error very often occurs on inspecting the pinhole defects. More specifically, the transmitted light beam would have to appear in number equal to the number of the pinhole defects. However, it has been confirmed that the number of the transmitted light beams has not been equal to the number of the pinhole defects and has been greater than the latter.